This invention is concerned with a low inertia induction motor which is so constructed that its speed responding characteristic at the time of its start and stop may be enhanced.
Generally speaking, in a servo motor for numerical control, or a driving motor for industrial sewing machines, or others, a steep rising or trailing performance is required in the rotational speed thereof for augmenting its responding characteristic at the start and stop.
For such use in industry, there has so far been employed in general a direct current electric motor driven by a thyrister-Leonard control power source. However, this d.c. motor has various disadvantages such that it necessitates a permanent magnet, a commutator, a commutating brush, and others, that it cannot increase its rotational speed so much from the standpoint of its commutating characteristic, that it has a limit against reduction in its size and weight, and that it requires careful, constant watch on and maintenance due to wear of the commutating brush, and various other disadvantages.
In contrast to this, since the induction motor can be operated in a maintenance-free fashion, it can take the place of the d.c. motor for the aforementioned use, provided that an inexpensive inverter control electric power source becomes available.
However, since the conventional induction motor includes in its rotor a yoke portion which constitutes a part of the magnetic pass for the rotating magnetic field, the rotor increases its weight, and the speed response thereof disadvantageously lowers due to increase in the inertial moment. As the means for improving such point of disadvantage, there have been adopted various methods such as one that uses a fly-wheel which is constantly driven by the motor to perform connection and disconnection betweeh the fly-wheel and the load by means of a clutch, so as to utilize the energy of rotation of the fly-wheel, and other methods. Even such measure, however, is not perfectly free from the disadvantages in that the power consumption is great, and the construction of the motor is complicated so as to invite an increase in the manufacturing cost.